Finely divided RuO2 /plastic matrix

ABSTRACT

Finely divided RuO 2  serves as an efficient and long-lived electrocatalyst when dispersed in a polymer matrix chemically and mechanically inert to an electrolyte. When applied to a substrate, generally an electrically conductive substrate, an electrode, particularly suited for use as an anode at which oxygen is evolved, is obtained. A coating of polyvinylidene fluoride containing RuO 2  having a particle size of less than 0.1 micron on a titanium substrate is exemplary.

BACKGROUND OF THE INVENTION

In the area of electrochemical reactions, those processes which employelectrodes functioning as "oxygen evolving anodes" are of considerablecommercial significance. Examples of such processes includeelectrowinning, e.g., the aqueous electrowinning of antimony, cadmium,chromium, zinc, copper and the like; water electrolysis, e.g., theevolution of oxgyen in a life support system; aqueous metal plating,cathodic protection in brackish waters, oxygen regeneration in waterpollution abatement, organic synthesis and others. An obviousrequirement of an anode for use in such processes is a low oxygenovervoltage, "overvoltage" referring to the excess electrical potentialover the theoretical reversible potential at which the desired elementis discharged at the electrode surface under equilibrium conditions.While a number of materials have been advanced for use as "oxygenanodes," such materials being alleged to have low oxygen overvoltages,these suffer in other ways which limit their application, such as beingchemically reactive, lacking dimensional stability, being of excessivecost, being sensitive to impurities in the system and others. Probablythe biggest problem, however, even with an electrode having the requiredlow overvoltage, is the short useful lifetime exhibited by suchelectrodes under commercial operating conditions. That is, whileexhibiting an initially low oxygen overvoltage, the operating voltagesteadily increases until the anode either fails to pass currentcompletely or does so only at economically unacceptable levels ofpotential.

STATEMENT OF THE INVENTION

Therefore it is an object of the present invention to provide anelectrode particularly useful as an oxygen anode and having a low oxygenovervoltage, chemical stability and a long useful life under commercialoperating conditions.

It is a further object of the present invention to provide a coatingmaterial for use on an electrically conductive substrate, which coatinghas the aforesaid advantages.

These and further objects of the present invention will become apparentto those skilled in the art from the specification and claims whichfollow.

It has now been found that finely divided RuO₂ having a particle size of.Iadd.0.5 to .Iaddend.0.1 micron or less dispersed in a chemically andmechanically inert organic polymer matrix exhibits a low oxygenovervoltage for a surprisingly long period of time. It has further beenfound that such a combination of finely divided RuO₂ and organic polymeron an electrically conductive substrate provides an efficient electrodefor many electrochemical reactions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The RuO₂ useful in the practice of the present invention is that havinga particle size of less than 0.1 micron. .Iadd.Other particle sizes from0.5 to 0.1 or less are useful but to a lesser extent. .Iaddend.Thecriticality of this extremely fine particle size is clearly demonstratedby the specific example which follows. There it is shown that both theoxygen over-voltage and lifetime of an anode employing the finelydivided material are dramatically improved, as compared to an evenslightly more coarse RuO₂.

The method of providing the RuO₂ in a finely divided form is criticalonly in that the requisite particle size be obtained. One such methodinvolves the dissolution of ruthenium trichloride in an acid solutionfollowed by treatment with a base to the alkaline side and finally byacidification to a pH within the range of 6-6.5. The resultingsuspension is washed thoroughly with distilled water until free of thesalt of neutralization, dried at 100° C. and fired at elevatedtemperatures, generally not in excess of 500° C. Other methods may beused, provided a relatively clean finely divided RuO₂ is obtained..Iadd.Other platinum group metal oxides such as oxides of rhodium,palladium, osmium, iridium and platinum may be used. .Iaddend.

The nature of the organic polymer with which the finely divided RuO₂ isto be used may vary greatly depending upon the intended application ofthe electrode. Obviously, since the organic polymer is intended to bindtogether and provide some mechanical support for the RuO₂, as well as toprovide adhesion to and protection for any underlying substrate, thepolymer itself must exhibit chemical and mechanical resistance to thesurrounding environment. For example, where the electrode may besubjected to elevated temperatures, use of a polymer having a highmelting or softening point is indicated. In corrosive environments,polymers having good resistance to the chemicals involved will beemployed. An electrode of varying susceptibility to wetting by theelectrolyte may be obtained by using more or less hydrophobic polymers.Thus it will be seen that many classes of polymers are useful in thepresent invention. Particularly useful, owing to their relative thermalstability and excellent chemical resistance, are the fluorocarbons suchas polyvinyl fluoride, polyvinylidene fluoride, polytetrafluoroethylene,polyhexafluoroethylene and polychlorotrifluoroethylene. Also useful arethe various acrylics, such as polymethyl methacrylate; phenolics, suchas phenolformaldehyde; polyethylene; polystyrene; polyacrylonitrile;acrylonitrile-vinyl chloride copolymer; polyvinylidene chloride;polyesters; polyimides; polymercaptans; polysulfones; polycarbonates andthe like.

The amount of RuO₂ .Iadd.or other platinum group metal oxide.Iaddend.used per part by weight of polymer varies with the applicationintended and the properties desired but generally is within the range of6-1:1. In most instances the RuO₂ /polymer composition will be used as acoating on a substrate, in which case the amount of coating applied persquare foot of substrate will likewise vary with the intendedapplication and also with the RuO₂ :polymer weight ratio. Obviously,since RuO₂ adds electrical conductivity and catalytic properties, thehigher the weight ratio the more conductive the resultant coating.Therefore thicker coatings may be applied of the lower resistance, highRuO₂ content coating.

The composition of the present invention may be applied to a number ofsubstrates depending upon the use to which the resulting electrode is tobe put. For example, if the RuO₂ content of the coating is high, theconductivity of the coating will be such that the substrate itself neednot be conductive, the coating serving as the sole vehicle for theconduction of current. In such instances inert materials, e.g., ceramic,will be quite useful as substrates since their only function is toprovide mechanical support for the coating. In most instances, however,where high current levels are employed it will be desirable andconvenient to use an electrically conductive substrate. Preferably sucha substrate will be inert to the electrolyte and products of theelectrolysis. Examples of such substrates are nickel, steel, the valvemetals (titanium, tantalum, zirconium and niobium), lead, lead-antimonyalloys, lead-thallium alloys and the like.

The physical form of the substrate, which in most instances determinesthe structure of the resulting electrode, may vary widely. While solidsheets, rods and the like are useful in many instances, often preferredis a mesh or expanded metal configuration. Such a substrate insures goodcoating adhesion and uniform current distribution.

The method of preparation of the desired electrodes is not critical tothe present invention. Any of the known techniques for causing a mixtureof a polymeric material with a filler (in this case the RuO₂) tosolidify, especially on a substrate, are useful. One such techniqueinvolves the preparation of a codispersion of the polymeric material andthe RuO₂, generally in a liquid medium to facilitate mixing andapplication, after which the dispersion is applied to the substratebeing used, e.g. by dip-coating or brushing. Subsequently the coatedstructure is heated, to flash off the liquid and cause some melt-flowingto occur, then cooled to yield the final electrode. An alternate andsomewhat preferred technique involves the preparation of a relativelyviscous paint-type formulation containing from 20-45 percent dissolvedpolymeric material plus disposed RuO₂ or other Pt group metal oxideconductive filler in a liquid organic vehicle. A substrate, especially amesh substrate, may then be coated with the "paint," for example bydipping into the solution, with curing, at temperatures on the order of.Badd.500° F. for 3- 20 minutes to evaporate the vehicle and solidifythe coating, between any number of successive layers, e.g., 1-5.

In view of the excellent electrical and electrolytic characterisitics ofthe electrodes of the present invention, the areas in which they willfind use will be limited primarily by mechanical considerations. Mostimportant of these considerations is that the use of the electrode notbe in an environment in which it may be expected that temperaturessubstantially exceeding the softening point of the polymer used in thecoating will be experienced since deformation and attrition may then beexpected. The aforementioned electrical and electrolytic propertiessuggest use in a variety of fields, for example:

1. as anodes in electrowinning of various metals, where oxygen is theprimary anode reaction product;

2. as oxygen-evolving anodes in water electrolysis systems for lifesupport applications;

3. as anodes in electroplating systems, where nonexpandable anodes withgood stability and low oxygen evolution potential are desirable;

4. as anodes for cathodic protection in brackish water, where stabilitytoward oxygen evolution is also required and,

5. as electrodes in electro-organic synthesis, where oxygen-evolvingcharacteristics are desirable.

In order that those skilled in the art may more readily understand thepresent invention, the following specific example is afforded.

EXAMPLE

Finely divided RuO₂ is prepared by dissolving RuCl₃ in 1.5M HCl(aqueous) solution to the extent of 150 grams per liter. Said solutionis treated with NaOH (20 percent by weight) until just alkaline thenacidified with HCl to a pH within the range of 6.0-6.5. The resultantsuspension is washed twice by decantation with distilled water followedby filtration on a Buchner funnel with additional (hot) water wash untilthe hydrated oxide is free of NaCl. Finally the oxide is thoroughlydried at a temperature of about 110° C. .Iadd.and fired at elevatedtemperatures generally not in excess of 500° C. .Iaddend.The particlesize of substantially all of the resultant RuO₂ is determined to be lessthan 0.1 micron, particles as small as 300 Angstroms being noted.

For comparison, RuO₂ is also prepared by heating RuCl₃ powder in anoxidizing atmosphere at a temperature of 450° C. until completeconversion is obtained. The particle size of the resultant RuO₂ isdetermined to be greater than 0.5 micron and is labeled "coarse."

For further comparison RuO₂ is prepared by extensive ball milling ofRuCl₃ powder followed by screening through a 100 mesh sieve and firingin an oxidizing atmosphere at 450° C. as above. The resultant RuO₂ isdetermined to have a particle size within the range of 0.2-0.5 micronand is designated "medium."

Test electrodes are fabricated employing each of the above by coatingonto an oxide-free titanium metal sheet as follows. Four parts .Iadd.byweight .Iaddend.of the RuO₂ is mixed with one part .Iadd.by weight.Iaddend.of polyvinylidene fluoride. The mixture is then dispersed at asolids level of 40 percent in 1-methyl-2-pyrrolidinone. The titanium iscoated by brushing the dispersion onto the substrate followed by curing10 minutes at 500° F. The resultant electrodes are employed as theanodes in 1M H₂ SO₄ at room temperature and an anode current density of3 amperes per square inch. Results appear in the following table.Potential is measured versus the standard calomel electrode and ananode's lifetime is considered terminated when the voltage exceeds 3.0V.

                  TABLE                                                           ______________________________________                                        Electrode                                                                              RuO.sub.2     Initial Half                                           (RuO.sub.2)                                                                            Particle Size Cell Potential                                                                             Lifetime                                  Designation                                                                            μ          (Volts)      (Hrs.)                                    ______________________________________                                        Coarse   >0.5          1.38         1-2                                       Medium   .[.0.5-02..]..Iadd.0.5-0.2.Iaddend.                                                         1.33         26.5                                      Fine     <0.1          1.31         228                                       ______________________________________                                    

From the Table is becomes apparent that not only is a substantialvoltage advantage obtained through the use of the finely divided RuO₂ ofthe present invention but, most importantly, the lifetime of the anodeis extended to a remarkable and unexpected degree.

While the invention has been described by reference to certain preferredembodiments by which it may be carried into effect, it is not to be solimited since changes and alterations may be made therein as is apparentfrom the scope of the appended claims.

I claim:
 1. A coating for use in electrolytic applications which coatingconsists essentially of a solidified intimate mixture of a substantiallychemically and mechanically inert organic polymer and a RuO₂electrocatalyst having a particle size of less than 0.1 micron.
 2. Anelectrode for use in electrolytic processes which electrode consistsessentially of a supporting substrate having a coating on at least aportion of the surface thereof which coating consists essentially of asolidified intimate mixture of a substantially chemically andmechanically inert organic polymer and a RuO₂ electrocatalyst having aparticle size of less than 0.1 micron.
 3. An electrode as in claim 2wherein the organic polymer is a fluorocarbon polymer.
 4. An electrodeas in claim 3 wherein the fluorocarbon polymer is polyvinylidenefluoride.
 5. An electrode as in claim 2 wherein the weight ratio of RuO₂:polymer is 6-1:1.
 6. An electrode for use in electrolytic processeswhich electrode consists essentially of an electrically conductivesupporting substrate having a coating on at least a portion of thesurface thereof which coating consists essentially of a solidifiedintimate mixture of a substantially chemically and mechanically inertorganic polymer and a RuO₂ electrocatalyst having a particle size ofless than 0.1 micron.
 7. An electrode as in claim 6 wherein theelectrically conductive substrate is selected from the group consistingof nickel, steel, titanium, tantalum, zirconium, niobium, lead,lead-antimony alloys and lead-thallium alloys.
 8. An anode for use inelectrolytic processes wherein oxygen is generated as the anode, whichanode consists essentially of,a. a supporting substrate selected fromthe group consisting of nickel, steel, titanium, tantalum, zirconium,niobium, lead, lead-antimony alloys and lead-thallium alloys, and b. acoating consisting essentially of a solidified intimate mixture of afluorocarbon polymer and a RuO₂ electrocatalyst having a particle sizeof less than 0.1 micron, the weight ratio of RuO₂ :polymer being from6-1:1.
 9. An anode as in claim 8 wherein the polymer is polyvinylidenefluoride. .Iadd.
 10. An electrocatalytic material consisting essentiallyof a solidified intimate mixture of a substantially chemically andmechanically inert organic polymer and an electrocatalyst of finelydivided RuO₂ having a particle size of less than 0.5 microns..Iaddend..Iadd.
 11. An electrocatalytic material consisting essentiallyof a solidified intimate mixture of a substantially chemically andmechanically inert organic polymer and an electrocatalyst of RuO₂ havinga particle size of approximately 0.1 to approximately 0.5 microns..Iaddend.